1. Field of the Invention
The present invention relates to a brushless DC fan driver, more particularly, to a brushless DC fan driver capable of utilizing anti-electromotive voltages to enhance the operational efficiency of the DC fan and prevent overshoot voltage problems.
2. Description of Related Art
FIG. 8 shows a conventional brushless DC fan driver for controlling a brushless DC fan that has two phase coils L1 and L2. The brushless DC fan driver has a Hall sensor (1), an amplifier (2) connected to the Hall sensor (1), a pulse generator (3) connected to the output of the amplifier (2) and two driving units (4, 5) connected to the pulse generator (3). The first driving unit (4) is composed of transistors Q11 and Q12 in Darlington connection. The second driving unit (5) is likely composed of transistors Q13 and Q14 in Darlington connection. A Zener diode Z1 is connected between the base and collector of the transistor Q11, while another zener diode Z2 is connected between the base and collector of the transistor Q13. The output terminals (6, 7) of the two driving units (4, 5) are connected to the two coils L1 and L2 of the fan. The two coils L1 and L2 are further connected to the power source VCC via a diode.
In use, when the fan is powered on, the pulse generator (3) generates a pulse signal to turn on the first driving unit (4) so that current is applied to coil L1 thereby generating a magnetic field to rotate a rotor (not shown) of the fan. Then, the Hall sensor (1) detects the change of magnetic polarity caused by the rotation of the rotor thereby generating a voltage signal to the pulse generator (3) via the amplifier (2). Consequently, the phase of the pulse signal generated by the pulse generator (3) is reversed to turn off the first driving circuit (4) and turn on the second driving circuit (5) thereby applying current to coil L2. Coil L2 generates a magnetic field to further rotate the rotor. Accordingly, the two driving units (4, 5) are alternatively turned on and turned off to apply current to the two coils L1 and L2, respectively, thereby continuously rotating the rotor.
Because anti-electromotive overshoot voltages may be generated by coils L1 and L2 at the instant when the current to coils L1 and L2 stops, the two Zener diodes Z1 and Z2 are provided to prevent such an overshoot voltage problem. The Zener diodes Z1 and Z2 bypass the overshoot voltages to protect the driving units (4, 5) from the overshoot voltages. However, instead of simply bypassing the overshoot voltages, it is desired to use the overshoot voltages to promote the operational efficiency of the fan.
Furthermore, in the standard CMOS configuration, Zener diodes are not available. Therefore, Zener diodes can not be integrated with a fan driver fabricated with a CMOS configuration. Consequently, the conventional fan driver is not satisfactory, and there is a need to have an improved fan diver which can mitigate and/or obviate the aforementioned problems.